Dielectric heating apparatus



Nov. 16, 1965 'r. LENART 3,218,429

DIELECTRIC HEATING APPARATUS Filed March 11, 1963 INVENTOR.

gunn fi United States Patent 3,218,429 DIELECTRIC HEATING APPARATUSTibor Lenart, Sundbyberg, Sweden, assignor to Aktieholaget Electrolux,Stockholm, Sweden, a corporation of Sweden Filed Mar. 11, 1963, Ser. No.264,398 Claims priority, application Sweden, Mar. 14, 1962, 2,804/62 6Claims. (Cl. 219-1055) My invention relates to dielectric heatingapparatus,

and more particularly to dielectric heating apparatus in which heatingof material is effected in a cavity of a heater by microwave electricalenergy developed by an oscillator, such as a magnetron, for example. Indielectric heating apparatus of this type, a magnetron often is employedfor developing ultra high frequency electrical energy as high as 2400 to2500 mc./s. The electrical energy, which is in the form of highfrequency electromagnetic waves, referred to as microwaves, istransmitted from the antenna of the magnetron through a wave guide tothe cavity of a heater or oven adapted to hold the load or material tobe heated. A part of the microwave electrical energy received in thecavity is refiected and transmitted back to the magnetron. This isobjectionable because the magnetron can be adversely affected when thereflected microwave electrical energy becomes sufliciently great. Forexample, the reflected electrical energy transmitted back to themagnetron effects objectionable heating of the anode. Further, themagnetron can be electronically damaged by the reflected electricalenergy for the reason that the magnetron and load in the cavity form .asystem equivalent to two resonant circuits which are coupled together.In the event the resonant frequency of the load is very close to theresonant frequency of the magnetron and the resistive component issmall, the magnetron can be forced to oscillate on the new frequency,which will overload the cathode and quickly damage the magnetron.

In order to prevent an excessive part of the microwave energy from beingreflected in the cavity and transmitted back to the magnetron, it isdesirable to match the impedance of the loaded cavity to that of themagnetron to produce the desired energy transfer from the magnetron tothe load. With a given useful load in the cavity, proper adjustment ofthe impedance of the loaded cavity to that of the magnetron can beeffected readily to obtain load matching. However, when the useful loadis removed from the cavity, the microwave energy reflected in the cavityand transmitted back to the magnetron increases and often exceeds avalue which adversely affects the magnetron and is objectionable.

The object of my invention is to construct dielectric heating apparatusof this type which will produce the desired energy transfer from themagnetron to the cavity under all operating conditions ranging from nouseful load to normal useful load, whereby the microwave energyreflected in the cavity and transmitted back to the magnetron willalways be within safe limits under all conditions encountered in use. Iaccomplish this by providing a dummy load which consists of a number ofdamping or energy absorbing bodies disposed in that part of a cavity inwhich the intensity of the microwave energy received from the magnetronis highest.

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My invention will be more fully set forth in the following descriptionreferring to the accompanying drawing, and the features of novelty whichcharacterize my invention will be pointed out with particularity in theclaims annexed to and forming a part of this specification.

In the accompanying drawing, the single figure is a diagrammaticrepresentation, in section, of a heating oven embodying my invention.

Referring to the drawing, I have shown my invention in connection with aheating oven having walls providing a cavity 1 including top and bottomsections 1a and 1b, respectively, separated by a horizontal partition 2desirably formed of material like glass, for example. A wall of the topcavity section 1a has a first apertured zone defining an access openingprovided with a door or closure member 3 movable between closed and openpositions and through which material to be heated is inserted andremoved from the topsection 1a. A receptacle 4 on the partition 2 may beemployed to hold the material which constitutes the useful load to beheated.

The useful load in the cavity is heated by microwave electricalenergy-produced by a magnetron 5 having an antenna 6 coupled thereto,the upper end or apex of as'a connectionbetween the magnetron 5 and thecavity above.

1, the connection being in communication with the cavity at a secondapertured zone 8 for introducing into the cavity microwave energysupplied by the oscillator or magnetron 5. A movable member 9 at one endof the wave guide 7 is provided with an operating element 10 which isaccessible in any suitable manner (not shown) to adjust the wave guide 7to any length desired. The cavity 1 under normal operating conditions isadapted to receive a normal useful load to be heated, as explained Thecavity 1, when the closure member 3 is moved to its closed position,defines a place of heating closed from the surroundings to preventpassage of microwave energy therefrom. This is so because, except forthe opening for the door 3 and the opening 8 for introducing microwaveenergy into the cavity 1, the walls of the cavity are imperforate.

A rotatable member 11 is positioned in the bottom cavity section 1b at aregion beneath the partition 2. The member 11 is fixed to the upper endof a vertical shaft 12 which projects downward exteriorly of the bottomcavity section 1b and is arranged to be driven by an electric motor 14.The microwave electrical energy received in the cavity in the form oftraveling electromagnetic waves produces an electromagnetic field whichis made as homogeneous as possible by'the rotatable member 11 whichfunctions as a stirrer.

In order that the microwave energy reflected in the cavity 1 andtransmitted back to the magnetron 5 will always be within safe limits,especially when there is no useful load in the cavity, an artificial ordummy load 15 is provided in the bottom cavity section 1b which isformed of material possessing ability to attract the electromagneticfield and also absorb microwave energy received in the cavity. The dummyload 15 comprises a plurality of elongated bodies 15a and 15b which aresupported in upright positions in the bottom cavity section 1b andextend upward from the bottom thereof. In the preferred embodiment shownand being described, the bodies a and 15b are in the form of cones andcylindrical rods, respectively, the lengths of the bodies being aboutonehalf of the length of the electromagnetic waves sent out or emittedby the magnetron 5.

Since dielectric heating involves the use of dielectric iosse to produceheat in poorly conducting materials, the bodies 15a and 15b desirablyare formed of a loss material, such as graphite, for example, and aninsulating material, such as a ceramic material, cement or a plastic,for example. When the bodies 15a and 15b are formed of materials of thiskind, the graphite can be mixed with a material selected from a ceramicmaterial, cement or a plastic, which serve as a binding agent in themixture. The bodies 15a and 15b also may be formed from cone-shaped andcylindrical-shaped metallic members coated with an enamel containingiron powder.

When the bodies 15:: and 15b in the bottom cavity section 1b aresubjected to the alternating current electromagnetic field produced bythe microwave energy transmitted from the magnetron 5, heating bydielectric losses results from the periodic stressing and displacementof the atoms of the material in the bodies acting as the dielectric of acapacitor. The loss is due to a property of dielectric material referredto as absorption, and the magnitude of the absorption in the bodies 15aand 15b can be regulated by controlling the proportion of loss andinsulating materials in the bodies.

Since the bodies 15a and 15!: are imperfect dielectrics, the presence ofboth loss material and insulating material is necessary. The expressionloss factor is often used to indicate the relative heating rates ofvarious dielectrics. When the bulk of a body 15a or 1512 is formed of aninsulating material in which only a relatively small quantity ofgraphite is present, the resulting mixture functions as a relativelysmall condenser having a relatively great loss factor. By varying theproportion of loss material and insulating material in a body 15a or151), the capacitance and loss factor of the body can be changed atwill. The capacitance of the bodies 15a and 15b determines their abilityto attract the electromagnetic field in the cavity, and the loss of thebodies determines what part of the concentrated electromagnetic energyin the bodies is utilized for the production of heat by dielectriclosses.

When a useful load, such as food, for example, is placed in the topcavity section 1a of the oven, the electrical energy is distributedbetween the food in the receptacle 4 and the artificial or dummy load15. The manner in which this distribution of electrical energy takesplace is dependent upon the formula wherein k is the dielectric constantof the material and l is the loss factor of the material. When food,which usually contains a relatively large quantity of water, is heatedin the oven, the product of k and If will be higher for the useful loadin receptacle 4 than for the artificial or dummy load 15 when graphite,for example, is employed as the loss material in the bodies 15a and1517. This is so because the dielectric constant k is considerablyhigher for water than for graphite and similar materials possessing thesame physical and electrical properties.

In the preferred embodiment, the useful load in the receptacle 4 isdisposed between the artificial or dummy load and the travelingelectromagnetic waves transmitted into the top cavity section 1a throughthe opening 8 in the wave guide 7. With this arrangement, the usefulload in the receptacle 4 will be subjected to direct radiation andeffective heating thereof will be promoted. In the absence of a usefulload in the top cavity section 1a, the bodies 15a and 15b forming thedummy or artificial load function as energy absorbing bodies which aredisposed in that part of the bottom cavity section 1!) in which theintensity and magnitude of the microwave energy received from themagnetron 5 is highest and the damping effect will be most effective toprotect the magnetron.

In view of the foregoing, it will now be understood that by providingthe bodies and 15b, the impedance of the loaded cavity is matched tothat of the magnetron to produce the desired energy transfer from themagnetron 5 to the cavity 1 under all operating conditions ranging fromnormal useful load to no useful load, so that the microwave energyreflected in the cavity 1 and transmitted back to the magnetron 5 willalways be within safe limits.

I claim:

1. Dielectric heating apparatus comprising an oscillator for supplyingmicrowave energy, an oven having Walls providing a cavity, a connectionbetween said oscillator and said cavity, said walls having first andsecond apertured zones and otherwise being imperforate, the firstapertured zone defining an access opening and a closure member thereformovable between closed and open posi tions, said connection being incommunication with said cavity at the second apertured zone forintroducing into said cavity microwave energy supplied by saidoscillator, said cavity when said closure member is moved to its closedposition defining a place of heating closed from the surroundings toprevent passage of microwave energy therefrom, said cavity under normaloperating conditions being adapted to receive a normal useful load to beheated, means disposed within the walls of said cavity to provide anartificial load for matching said cavity to said oscillator in theabsence of a useful load in said cavity, said artificial load meanscomprising damping body structure disposed within that part of saidcavity in which the damping effect will be effective to protect saidoscillator, and said damping body structure comprising a plurality ofdamping bodies, the length of said damping bodies being substantiallyone-half the length of the microwaves supplied by said oscillator.

2. Apparatus as set forth in claim 1 in which said damping bodies are inthe form of rods.

3. Apparatus as set forth in claim 2 in which said rods are cone-shaped.

4. Apparatus as set forth in claim 2 in which said rods are cylindrical.

5. Dielectric heating apparatus comprising an oscillator for supplyingmicrowave energy, an oven having walls providing a cavity, a connectionbetween said oscillator and said cavity, said Walls having first andsecond apertured zones and otherwise being imperforate, the firstapertured zone defining an access opening and a closure member thereformovable between closed and open positions, said connection being incommunication with said cavity at the second apertured zone forintroducing into said cavity microwave energy supplied by saidoscillator, said cavity when said closure member is moved to its closedposition defining a place of heating closed from the surroundings toprevent passage of microwave energy therefrom, said cavity under normaloperating conditions being adapted to receive a normal useful load to beheated, means disposed within the walls of said cavity to provide anartificial load for matching said cavity to said oscillator in theabsence of a useful load in said cavity, said artificial load meanscomprising damping body structure disposed within that part of saidcavity in which the damping effect will be effective to protect saidoscillator, and said damping body structure comprising a plurality ofdamping bodies which are formed from graphite and an insulating materialselected from a group consisting of ceramic, cement or plastic.

6. Dielectric heating apparatus comprising an oscillator for supplyingmicrowave energy, an oven having walls providing a cavity, a connectionbetween said oscillator and said cavity, said walls having first andsecond apertured zones and otherwise being imperforate, the firstapertured zone defining an access opening and a closure member thereformovable between closed and open positions, said connection being incommunication with said cavity at the second apertured Zone forintroducing into said cavity microwave energy supplied by saidoscillator, said cavity when said closure member is moved to its closedposition defining a place of heating closed from the surroundings toprevent passage of microwave energy therefrom, said cavity under normaloperating conditions being adapted to receive a normal useful load to beheated, means disposed within the walls of said cavity to provide anartificial load for matching said cavity to said oscillator in theabsence of a useful load in said cavity, said artificial load meanscomprising damping body structure disposed Within that part of saidcavity in which the damping effect will be effective to protect saidoscillator, 1

and said damping body structure comprising a plurality 6 of dampingbodies which are formed of metal coated with an enamel containing ironpowder.

References Cited by the Examiner UNITED STATES PATENTS 2,207,845 7/1940Wolff 333-34 2,584,162 2/1952 Sensiper et al 333-34 2,704,802 3/1955Blass et 'al 219-1055 2,804,598 8/1957 Fano 333-34 2,820,127 1/1958Argento et al 219-1055 2,827,537 3/1958 Haagensen 219-55 2,961,52011/1960 Long 219-1055 2,977,591 3/1961 Tanner 333-81 RICHARD M. WOOD,Primary Examiner.

1. DIELECTRIC HEATING APPARATUS COMPRISING AN OSCILLATOR FOR SUPPLYINGMICROWAVE ENERGY, AN OVEN HAVING WALLS PROVIDING A CAVITY, A CONNECTIONBETWEEN SAID OSCILLATOR AND SAID CAVITY, SAID WALLS HAVING FIRST ANDSECOND APERTURED ZONES AND OTHERWISE BEING IMPERFORATE, THE FIRSTAPERTURED ZONE DEFINING AN ACCESS OPENING AND A CLOSURE MEMBER THEREFORMOVABLE BETWEEN CLOSED AND OPEN POSITIONS, SAID CONNECTION BEING INCOMMUNICATION WITH SAID CAVITY AT THE SECOND APERTURED ZONE FORINTRODUCING INTO SAID CAVITY MICROWAVE ENERGY SUPPLIED BY SAIDOSCILLATOR, SAID CAVITY WHEN SAID CLOSURE MEMBER IS MOVED TO ITS CLOSEDPOSITION DEFINING A PLACE OF HEATING CLOSED FROM THE SURROUNDINGS TOPREVENT PASSAGE OF MICROWAVE ENERGY THEREFROM, SAID CAVITY UNDER NORMALOPERATING CONDITIONS BEING ADAPTED TO RECEIVE A NORMAL USEFUL LOAD TO BEHEATED, MEANS DISPOSED WITHIN THE WALLS OF SAID CAVITY TO PROVIDE ANARTIFICIAL LOAD FOR MATCHING SAID CAVITY TO SAID OSCILLATOR IN THEABSENCE OF A USEFUL LOAD IN SAID CAVITY, SAID ARTIFICIAL LOAD MEANSCOMPRISING DAMPING BODY STRUCTURE DISPOSED WITHIN THAT PART OF SAIDCAVITY IN WHICH THE DAMPING EFFECT WILL BE EFFECTIVE TO PROTECT SAIDOSCILLATOR, AND SAID DAMPING BODY STRUCTURE COMPRISING A PLURALITY OFDAMPING BODIES, THE LENGTH OF SAID DAMPING BODIES BEING SUBSTANTIALLYONE-HALF THE LENGTH OF THE MICROWAVES SUPPLIED BY SAID OSCILLATOR.